CCK date field in user profile (but hidden on registration form) results in array being passed to check_plain()

I have a "membership valid through" CCK date field in the user profile, which defaults to the end of the current year. The field is

This field is not user-editable, and is "hidden" on the registration form. This results in a hidden input control being created, but the value of that hidden input control is being passed through to the theme as an array (month/day/year). Hidden input controls must have text #values in order to get onto the form properly.

This results in check_plain() choking on the htmlspecialchars() call.

I presume this is a Date issue, but admittedly I'm not sure where to look in the chain for problems.